Power converters may include a large transistor that is rapidly switched on and off. This rapid switching can cause noise on the resulting power-supply voltage, and upon other nearly signals. Downstream and upstream devices from the power converter can generate emissions that create disturbances in other electronic equipment. For example, a noisy switching power supply driving a portable computer may emit radiation that interferes with a television receiver. Intentional receivers, such as televisions, cellular phones, pagers, and wireless devices, are often affected by unintentional transmitters that emit electromagnetic radiation. As both types of electronic equipment become more common, interference becomes more noticeable to the consumer.
Electromagnetic interference (EMI) is a measure of the amount of interference that an electronic device (the unintentional transmitter) disturbs an intentional receiver. Government agencies such as the Federal Communication Commission (FCC) strictly regulate the amount of radiation or EMI that an electronic device can generate.
Improving technology also worsens the EMI problem. Faster clock rates of higher-speed equipment generate more radiation. Higher resolution monitors and displays require that more pixels be transferred to the screen for each screen refresh period; thus a higher clock rate and more interference results.
Traditional techniques to reduce EMI attempt to contain radiation or to reduce the amount of radiation generated. Coax wires and shielded cables are effective at containing radiation, but are expensive, heavy, bulky, and inflexible. The weight and bulk of shielded cables make them undesirable for portable devices. Metal chassis with sealed seams are effective for reducing EMI of desktop equipment, but portable devices are kept light by using plastic.
Lower voltages reduce the intensity of the radiation generated, and the new 3-volt standard has helped reduce EMI at all harmonics. Proper impedance matching and termination of signals reduces ringing and harmonics, and shorter signal traces further reduce radiation. Ground planes on PCB's or ground lines running parallel with signal lines effectively shield signals on boards. Filtering can reduce sharp rise and fall times and reduce radiation by wave shaping since more sinusoidal waves have fewer harmonics than square waves. Of course, filters require additional capacitors, resistors, or inductors, raising the cost. All of these techniques are useful to varying extents.
Large physical components are often needed as filters to reduce noise and EMI. For example, a large inductor coil may be added to a power converter, along with high-value capacitors that also are bulky. Precision capacitors or resistors may be needed. These bulky components are undesirable and costly and defeat integration.
A newer technique to reduce EMI is to vary or modulate the frequency of clocks. This technique known as spread spectrum, since the frequency spectrum of the clock is spread out over a wider range of frequencies. FIG. 1 shows a graph of radiation intensity as a function of frequency for an un-modulated clock signal. A sharp spike occurs at a harmonic of the clock's frequency, 40 MHz. Since the clock constantly operates at the rated frequency, all of the energy of the radiation appears in a narrow spike, which has a large amplitude. The spike has an amplitude over the EMI limit set by the FCC. The high intensity of the spike can cause interference in a receiver.
FIG. 2 is a graph of radiation intensity as a function of frequency generated by a modulated clock. The clock's frequency is not constant, but is varied with time over a range of +5% to −5% of the rated frequency. Thus the clock operates at 40 MHz for a period of time, but also operates at other frequencies between 38 MHz and at 42 MHz at other times. Such a clock can be generated by slowly changing the frequency from 38 MHz to 42 MHz and then slowly reducing the frequency back to 38 MHz. A voltage-controlled oscillator (VCO) can be used with the input voltage being slewed back and forth between voltages that generate 38 MHz and 42 MHz oscillations.
Since the modulated clock spends only part of the time at 40 MHz, the intensity of the radiation, averaged over a relatively long time, is reduced. The total energy of the radiation at all frequencies is about the same as for the un-modulated clock of FIG. 1, but the intensity at any particular frequency is greatly reduced. Interference at any one frequency is reduced since receivers generally are tuned to a particular frequency (even FM receivers are tuned to a relatively small range of frequencies).
Thus modulating the clock's frequency reduces the maximum intensity of radiation at any one frequency, although the energy radiated at all frequencies is not reduced. This has the practical effect of reducing interference for receivers tuned to a fixed frequency.
Long Sweep Period of Modulation—FIG. 3
FIG. 3 is a graph of a modulated clock's frequency as a function of time over a few sweep periods. The clock's nominal frequency is 40 MHz. The clock is modulated by +/−5%, from 38 MHz to 42 MHz. The clock's frequency is swept from minimum to maximum frequencies over one or two thousand clock periods so that adjacent clock pulses have a very small variation. A 40 MHz clock with a 25 nanosecond (ns) period is varied from 26.25 ns to 23.75 ns over a sweep period, a variation of +/−1.25 ns. A 37 KHz sweep rate has a sweep period of 27 micro-seconds (μs). A sweep period is 27 μs/25 ns or 1081 clock periods. The cycle-to-cycle period variation for two adjacent clock periods is thus 5 ns/1081 or 4.62 pico-seconds (ps). The sweep frequency is typically 15 to 50 KHz.
Such spread spectrum using frequency dithering is effective in reducing EMI. Bulky filter components such as large capacitors and inductors are not needed. However, often such frequency dithering requires switching precise components such as precision capacitors to generate small steps or adjustments in frequency. Matching of these precision capacitors or resistors is difficult. Very large capacitors or very small currents or resistors must be used. Such small currents are themselves subject to noise interference and leakages and are thus undesirable.
What is desired is a frequency dithering circuit for reducing EMI for a switched power supply. A dithering circuit that does not need bulky filters or precision components is desirable. A dithering circuit that can be integrated with other circuits for a small form factor is desirable. A dithering circuit that can be tuned or programmed for the degree of frequency dithering is desirable. A dithering circuit that can be used with an oscillator for a switched power supply is desirable. A dithering circuit that can be used as a module for a clock generator is also desired.